The fundamental goal of the proposed studies is to identify the genetic and molecular requirements for secretion of the non-lytic bacteriophage CTXphi from Vibrio cholerae. CTXphi is the only filamentous phage known to play a role in horizontal transfer of virulence genes among bacteria: it transmits genes encoding cholera toxin, the principal virulence factor of V. cholerae. Unlike other filamentous phages, production of CTXphi is dependent upon at least one component of a chromosomally encoded secretion system. Thus, the specific aims of this proposal include i) determination of the full extent of chromosomal secretion system proteins required for CTXphi secretion; ii) mutational and biochemical analyses of a key phage protein that mediates phage secretion, and analysis of interactions between this protein and others required for secretion, and iii) identification, via a genetic screen, of additional chromosomal genes required for CTXphi secretion. These studies should further understanding of the processes of phage and protein secretion and illuminate a novel relationship between them. It may ultimately aid in generation of new live attenuated V. cholerae vaccines and contribute to development of potent antimicrobial agents.